Dry conversion of high purity ultrafine silicon powder to densified pellet form for silicon melting applications

ABSTRACT

A method for making bulk silicon material consisting of silicon pellets for making silicon ingots from an agglomerate-free source of high purity, ultra fine silicon powder includes feeding a controlled amount of silicon powder into a pellet die and dry compacting the powder at ambient temperature with pressure to produce a pellet that has a density of about 50-85% of the theoretical density of elemental silicon, a weight within a range of about 1.0 gram to about 3.0 grams, a diameter in the range of 10 mm to 20 mm and preferably of about 14 mm, and a height in the range of 5 mm to 15 mm and preferably of about 10 mm.

RELATED APPLICATIONS

This application is a divisional application to pending application Ser. No. 11,668,488 filed Jan. 30, 2007. Application Ser. No. 11/668,488 is a continuation-in-part to application Ser. No. 10/413,774, filed Apr. 15, 2003. Application Ser. No. 10/413,774 claims the benefit of U.S. Provisional Patent Application Ser. No. 60/372,980, filed Apr. 15, 2002, under 35 U.S.C. §119(e).

FIELD OF INVENTION

This invention relates to raw materials used for melting to make silicon ingots, in particular to a bulk silicon material consisting of silicon pellets make by compacting silicon powder without additives or binders under pressure at ambient temperatures into pellet form.

BACKGROUND OF THE INVENTION

The crystalline (mono-crystal or multi-crystal) silicon materials used for semiconductor as well as for photovoltaic devices manufacturing are produced by crystal growth from melt. The feedstock for the silicon crystal growth process is high purity silicon fabricated or produced by high temperature decomposition of silicon containing chloride (such as trichlorosilane or mono-silane).

The forms of the silicon feedstock that come directly from silicon manufacturers are generally chunks and chips (from the breaking down of large silicon rods) and silicon beads (size ranges from several hundred micrometers to millimeters). Such feedstock can be packed well into the crucibles that are used for melting the silicon charges with a packing factor of more than 50%.

The silicon beads are generally made by a fluidized bed process. This process also produces ultra-fine silicon powder (size ranges from sub-micron to several hundred micrometers) as a byproduct in addition to the useful silicon beads. The ultra-fine silicon powder is normally referred as Cyclone powder or filter powder depending on where it is deposited; the range of sizes for the two powders is also different. So far there is no effective way to utilize this ultra-fine powder hence the effective silicon conversion yield of the fluidized bed reactors is low.

Such ultra-fine silicon power has a purity as high as that used for the crystal growth. However, because of its submicron size, it has a bulk density of about 0.25-1 gm/cc which is significantly low when compared to the silicon solid density of 2.33 gm/cc. Because of the loosely bound nature of the powder the crucible cannot be loaded with more powder. For example, a 69×69×42 cm crucible can hold up to 300 Kg of solid silicon, while the powder can be charged only up to a maximum of 150 Kg. Conventional melt replenishment by continuous feeding of chips or beads is also not possible with the above said powder because of the loosely bound nature. Lack of proper way to utilize the ultra-fine silicon powder renders such by-products of much less value. Furthermore, the low bulk density presents storage problems due to requirements of large space.

Such ultra-fine silicon powder can also be formed by homogenous thermal decomposition of silicon containing gases (such as mono-silane). This homogenous decomposition is a much cheaper process comparing to the heterogeneous deposition process used in the Siemens and the fluidized bed reactors. Therefore, a practical method of charging and feeding the ultra-fine silicon powder would have a significant impact on the cost of manufacturing the silicon feedstock. This is especially important for photovoltaic applications where cost reductions will make this renewable energy source viable for terrestrial applications.

Another problem associated with ultra-fine powders is the very large surface area; this results in an oxide coating on the powders. When such powders are heated to melt temperatures in ingot growth furnaces this presents problems in melting, reactions with the hot zone and degradation in performance of the silicon when used for solar cells. Therefore, ultra-fine powder additions to ingot growth charge have been limited to approximately five percent (5%).

The rapid growth of photovoltaic industry and current severe shortage of silicon feedstock has forced the manufacturers of crystalline silicon to use all sorts of silicon feedstock in all kinds of forms. This includes the mixing of silicon powder, generally of larger than 100 micrometers, with other forms such as chunks of silicon. However, the use of the ultra-fine silicon powder (sub-micron size) remains a challenge due to its very low bulk density. Moreover, the ultra-fine silicon can easily flow with gases, which make it very hard to handle. Typically, the first operation in an ingot growth furnace after loading the charge is evacuation of the chamber; this can result in sucking the ultra-fine powders out of the crucible.

Compacting of silicon powder has been mentioned in several publications, as listed in the references. Both Möller's paper, Sintering of Ultrafine Silicon Powder, and Takatori's paper, High Pressure Hot-Pressing of Silicon Powders, were specific sintering, i.e., with elevated temperature to achieve full densification. Both papers are focused on the grain boundaries formed after the high temperature sintering due to their interests in the mechanical properties of the formed bulk silicon materials.

In Möller's paper, the starting silicon powder has sub micron particle sizes in the order of 0.02 to 0.1 μm. A pre-shaped compaction of silicon powder, of little or no description, was made and subjected to high temperature sintering without pressing. The densification and microstructure development (grain size, grain boundary transformation, etc.) were investigated under different sintering conditions for purposes other than those of this invention.

In Takatori's paper, the compaction of silicon power was achieved by applying pressure and elevated temperature simultaneously. As known to the materials researchers, high temperature helps bulk diffusion as well as the grain-boundary diffusion, and thus helps the densification of the sintered material. In fact, as reported in this paper, sintered density of close to 100% of theoretical was achieved at temperature above 677° C. when combined with high pressing pressure (>1 GPa).

What is needed is a cost effective method and product for reclaiming ultra fine silicon powder of high purity for uses requiring silicon of a high degree of purity.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a viable and practical process and machinery to convert otherwise unusable ultra fine silicon powder into a form factor, typically pressed pellets, that can be utilized in silicon melting processes. It is a further object to provide a process and machinery that will maintain the purity of the silicon to nearly the same level as the starting powder, either in the pellet form directly or in the consumption of the pellet for its intended purpose.

It is another object of the invention to provide a system and facility for conducting a powder-to-pellet conversion on a commercially useful production rate, such as high speed pellet pressing of up to 600 pellets or more per minute, and processing of up to 50 kg or more of silicon powder per hour.

It is an object of the compacting of ultra-fine silicon powder to enhance the packing factor of the silicon feed material in a crucible through the use of compacted silicon pellets.

Another objective of compacting the ultra-fine silicon powder is to enhance the thermal conductivity of the powder so that the silicon feed stock can be easily heated and melted.

It is an object of the present invention to provide a process for the production of pellets having the structural integrity to withstand packaging, handling and further processing by compressing silicon powder at an effective force, according to one embodiment, about approximately 10,000 N (1.12 US Tons) to achieve adequate adhesion between silicon particles.

One requirement for the resulting silicon pellets (compacts) is purity. The purity of the silicon in pellet form has to be maintained to nearly the same level as the starting powder in order for the pellets to be useful for making high purity silicon crystals. The silicon crystals are generally used for making semiconductor devices and solar cells. In some applications, a small amount of binder may be tolerable. In other application, small amounts of binder may be removed during consumption of the pellet.

One aspect of this invention is the development of a process that adds significant value to the silicon dust (ultra-fine powders) byproduct from the Fluid Bed and other processes.

The universal method of treating fine powders is by addition of organic and inorganic binders to convert the powder to granules and pellets, as are practiced in ceramics industry, certain powder metallurgical industry, and pharmaceutical industry. These processes add difficult-to-remove ingredients to the compact, and which in some processes are just contaminants to the material.

However, in one aspect of the process of this invention, silicon is pressed dry without addition of any outside material and at ambient temperature. It thereby keeps the purity of the pressed pellet very close to that of the starting material. It is the combination of the ability to convert silicon dust into compressed form by a dry no-binder technique that enables subsequent value-added use of the by-product silicon powder, for example in the silicon melting processes of photovoltaic and electronic applications.

The only other application of dry pressing of powders is in the manufacture of nuclear fuel oxide pellets by the MOX process. Even in this process small quantities of zinc stearate are utilized as an additive to provide for initial agglomeration and pellet strength while also serving as a lubricant in the pressing operation. It is removed in the subsequent high temperature sintering step.

In another aspect of the invention, a binder may be introduced into the powder to facilitate the forming of the pellets. Some or all of the binder may be removed during the pellet forming process, as by the heat of compression or the addition of a small amount supplemental heat.

Compacting fine powders at an elevated temperature (sintering) is another general practice for obtaining densified components. Sintering at high pressure (hot press) can achieve close to 100% density material and the fine grains obtained from hot pressure provide the needed structural strength. However, the application of the disclosed process has minimal structural requirements and the grain boundary is not important for the resulting pellets. Additionally, in an elevated temperature environment, impurities from the die can diffuse into the formed silicon pellets, and thus contaminate the feed stock. During heat up to high temperatures the oxide layer on ultra-fine powders can lead to reactions leading to silicon carbide formation, thereby resulting in contamination and degradation in performance.

One process of this invention utilizes binder-less cold pressing of silicon powder without additives to form pellets that can be utilized in subsequent silicon processes. However, an organic binder may be used in some cases, and be all or partially evaporated during or after forming. The exact pellet shape and size, or uniformity of shape and size, is not of particular importance except as may be dictated by the available pressing machinery. The structural integrity of the pellets needs only be sufficient to tolerate packaging and handling. There are no published references known to the Applicants that purport to utilize a process for effective use of otherwise unusable silicon dust.

One aspect of the invention is that the material purity may be maintained in the powder-to-pellet conversion operation, so long as the process is a binder-less dry method. In another aspect of the invention, any remaining binder may be removed in the consumption of the pellets, so that the resulting silicon product is of high purity not withstanding the use of binder in the feed stock pellets.

The form factor of the pellet is important, because the pellets have significantly less surface area as compared to the ultra-fine powders and, therefore, do not cause significant reactions during heat up. This minimizes contamination and degradation in performance and allows larger proportions (up to 100%) in the charge for ingot growth.

This invention is important because it provides a method to convert high purity, but otherwise wasted, ultra fine silicon powder into a form that is transportable in bulk, pure, and usable as feed material to silicon melting processes, and in particular those processes requiring high purity silicon.

In one aspect of the invention, the ultra fine silicon powder is transferred into a clean feed hopper attached to a pellet press machine such as a high quality Courtoy-type rotary indexing die and punch machine. Controlled quantities of the powder are fed into the die by use of an appropriate powder feeder. Since the powder is ultra fine, a special powder feeder may be required. The powder is pressed by the punch with a press force of several tons. No binder or additive is necessary to the process, although for some applications an organic binder may be used in the initial forming stage, and subsequently be evaporated during or after the pellet is formed.

The pressed pellet is ejected into a clean collection bin and transferred into a lined shipping container. The pelleting machinery is equipped for automated and controlled operation. In addition, the entire process zone is located inside a controlled enclosure to maintain process and environment quality. The process facility also provides controlled ingress and filtered egress for environmental safety.

Since no binder or additive is required in converting the ultra fine silicon powder to pressed pellet form, only nominal pellet strength, satisfactory for the purpose of compaction and transfer to secondary operations, is necessary. Some surface dusting and occasional breakage of pellets still provides an acceptable yield of the high-value end product, the dry, high purity silicon pellets.

In other and various aspects of the invention, there is no minimum particle size for the powder. A small amount of binder may be added to the powder to facilitate forming of the pellets. A small amount of supplemental heat may be applied during the pellet forming process, during which some or all of the binder may be evaporated. What binder remains may be partially or fully evaporated during heating of the pellets for consumption, so that the purity of the silicon product is affected only slightly if at all.

The present invention is a method for making a silicon pellet. The method includes providing a source of high purity silicon powder having a median particle size within a range of up to about 100 micrometers and a range of particle size of not greater than about 1000 micrometers, feeding a controlled amount of said powder from said source into a pellet die, compacting with pressure said controlled amount of said powder in said die thereby forming a dry pellet of high purity silicon, and discharging said pellet from said die.

In some embodiments, the compacting is done at about ambient temperature. In some embodiments, the powder is free of intentional additives and binders. In other embodiments the powder includes an organic binder, and the compacting with pressure includes adding sufficient heat whereby the binder is evaporated.

And in certain embodiments the step of dry compressing includes applying a force greater than or equal to about approximately 10,000 Newton's to the powder.

The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is susceptible of many embodiments. As listed in Table 1, the results of one of several tests conducted by the Applicant, show that an ultra fine silicon powder of median particle size of about 13 micrometers, bulk density 0.56 g/cc (grams per cubic centimeter) and tap density about 0.68 g/cc, is converted into pellets of size 14 mm (millimeter) diameter×10 mm height, with a pellet weight of 2.3±0.05 g. The pelleting data for Table 1 was taken from pellets made with a clean Courtoy R 53 rotary multi-station pellet press with compression force capacity of up to 14 tons. The tool set is made out of Tungsten Carbide.

TABLE 1 Pellets Pellets Pellets Pellets 1 2 3 4 Starting powder 13.5 μm 3.97 μm 4.97 μm 11.5 μm median size Powder bulk density 0.56 0.58 0.65 0.34-0.58 (g/cc) Pellet diameter 14 18 18 12 (mm) Pellet height  9.5-10.0 5.3 5.3 4.1 (mm) Pellet density 1.49-1.58 1.41 1.41 1.72 (g/cc) Pellet density (% of 64-68% 60% 60% 74% bulk Si) Compression force 10,000 10,000 10,000 10,000 (N) Pellet purity NA NA NA GDMS, Close to powder

Table 1 illustrates silicon pellets made with different powder sizes and pellet sizes. The actual pellet size is not critical, and uniformity in size is not critical. In one embodiment, a precise quantity by weight of the silicon powder is fed into the die as a unit charge, and progressively compressed by a matching punch to the required force to achieve the pre-calculated dimension that represents the desired final pellet density based on the weight of the unit charge of powder fed into the die. Alternatively, the process may be operated on the basis of compressing a unit charge that is sized as a pre-calculated volume of powder, and/or compacting the unit charge to a pre-calculated final pressure or volume. The process criteria may be selected by calculation and/or by trial and testing of a useful range of the listed variables in Table 1 to achieve the desired pellet product.

To produce silicon pellets having the mechanical integrity to withstand packaging, handling, storage and further processing, the silicon powder must be exposed to an effective pressure to insure that there is adequate adhesion between the silicon power particles. By compressing the powder, the silicon atoms of different particles are in sufficiently close proximity to permit bonding or attraction between the atoms and as a result, the particles. The compression packs the particles closer together, eliminating voids and holding the particles, and the atoms of which they are comprised, in close proximity. While this packing need not be the highly ordered packing of crystalline silicon, higher degrees of order and of bonding produce more mechanically stable and purer pellets. According to one embodiment a force of about approximately 10,000 N (1.12 US Tons) was used. Those skilled in the art will readily appreciate that the forces in excess of 10 KN would likewise produce pellets of sufficient structural integrity, as would weaker forces so long as the particles are compacted sufficiently to adhere without additives.

For example, the starting power particle size may be expected to range up to 20 μm and more. The power density may range from less than 0.60 g/cc to more than 0.75 g/cc. The compressive force, exact pellet geometry and volume, and final pellet density may be a function of the available machinery, but testing suggests that a good pellet can be produced from the range of powder specified, with 10,000-20,000 Newton's in a volume range of about 0.5 to 2.6 cubic centimeters, weight range of about 1.0-3.0 grams, and a density range of about 50-75% of the theoretical density of elemental silicon. Extrapolating our test results, it is expected that an average pellet density of at least 50% will be necessary to survive bulk packaging and handling. Densities as high as 85% are expected to be attainable with smaller particle sizes and higher compression forces. Our test results demonstrated that the smaller the size of the powder, the better integrity of a formed pellet, or the easier for compacting.

As another example, For example, the starting power medium particle size may be expected to range up to 20 μm and more. The power density may range from less than 0.60 g/cc to more than 0.75 g/cc. To the silicon powder there may be added an organic binder. Organic binders such as Acetone, or Alcohol (methyl, ethyl or iso propyl) or isobutyl methacrylate mixed with ethylene dichloride and carbon tetra chloride, and others, may be used to facilitate forming.

The organic binders such as acetone or alcohol provide a good binding force between the silicon particles and easily get evaporated at room temperature without leaving any significant residues.

Other binders such as cellulose ethers (solid binder or added with alcohol solvent) need a high temperature bakeout to burn off the binder. The time for weight loss upon heating can be determined by differential thermal analysis. For cellulose ethers the complete burn off occurs at 400 degrees centigrade, cleanly and predictably with first order kinetics. All binders added should be of electronic grade.

The compressive force, exact pellet geometry and volume, and final pellet density may be a function of the available machinery, but testing suggests that a good pellet can be produced from the range of powder specified, with 10,000-20,000 Newton's in a volume range of about 0.5 to 2.6 cubic centimeters, weight range of about 1.0-3.0 grams, and a density range of about 50-75% of the theoretical density of elemental silicon. Extrapolating our test results, it is expected that an average pellet density of at least 50% will be necessary to survive bulk packaging and handling. Densities as high as 85% are expected to be attainable with smaller particle sizes and higher compression forces. Our test results demonstrated that the smaller the size of the powder, the better integrity of a formed pellet, or the easier for compacting.

According to one embodiment of the present invention dry silicon powder is used. The presence of moisture might result in clumping, or impurities that might have a deleterious effect on the purity of the final product, the integrity of the pellet, or the operation of the die. Those skilled in the art would, however, readily appreciate that the use of wet silicon powder would be within the scope of the invention.

The compressed pellets are ejected from the machine through the take-off system. The pellets provide a loose bulk material form of silicon for melting for high purity silicon requirements. The powder-to-pellet conversion is accomplished dry, with no added ingredient in the process. This is required to maintain the purity of the silicon for subsequent use.

The term “pellet” is herein inclusive of any form factor and descriptive term that implies a compacted small volume of the raw material, the pellets produced in quantity in the nature of a loose granular bulk material that facilitates easy handling methods and ready conformance to container shapes.

The invention is further extended by the utilization of the pelletized dry silicon in the making of high purity silicon ingots. The suitability of the high-purity dry-compacted pellets for melting into high purity silicon ingots is effectively demonstrated by the method of one embodiment of containing the pellets in a fused quartz crucible, baking in vacuum at 1350° C., and then melting in an inductively heated graphite susceptor system as is well known in the art. The melt is taken up to 1600° C., and then cooled. The resulting ingot is very bright and shiny, with no inclusions in it. There may be a trace of residual oxide material as slag on top center of the melted ingot. Dry silicon pellets, pressed from silicon powder, were first melted to form an ingot of high purity silicon on Jan. 28, 2002.

The basic steps of another embodiment method for making high purity silicon pellets is as follows: providing a source of high purity silicon powder, feeding the powder into a blender, operating the blender to remove agglomerates, discharging the powder into a hopper, feeding a controlled amount by weight or volume of the powder into a die, dry compacting the powder with pressure, exclusive of any additives or wetting agents, and then discharging the dry pellet from the die. The machinery may be configured to operate multiple lines of multiple dies, to meet high volume requirements.

The further steps of this embodiment of making high purity silicon ingots from the dry silicon pellets is conventional except for the use of the dry silicon pellets and the resulting purity of the ingots: containing a suitable number of the silicon pellets in a fused quartz crucible, baking the crucible with pellets in vacuum at about 1350 degrees C., melting the pellets at up to about 1600 degrees Centigrade in an inductively or resistively heated graphite susceptor system, and cooling the melt so as to produce an ingot.

In the disclosed invention, in distinction to the work of Möller and Takatori discussed previously, the grain boundaries of the pellets is of less importance because the formed silicon shapes are to be used as feed stock for melting. Therefore, no sintering is necessary as the disclosed process can obtain reasonably aggregated silicon pellets. High temperature sintering may introduce impurity into the silicon pellets, which is highly prohibited in the application of the disclosed process. Also, the invention disclosed does not require elevated temperatures, although supplemental heat may be added in some cases. The achieved densification is in the range of 60-75% of theoretical, which has been shown to be dense enough for the designated applications.

The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. 

1. A method for making a silicon pellet comprising: providing a source of high purity silicon powder having a median particle size within a range of up to about 100 micrometers and a range of particle size of not greater than about 1000 micrometers; feeding a controlled amount of said powder from said source into a pellet die; compacting with pressure said controlled amount of said powder in said die thereby forming a dry pellet of high purity silicon; and discharging said pellet from said die.
 2. The method for making a silicon pellet according to claim 1, said compacting being done at about ambient temperature.
 3. The method for making a silicon pellet according to claim 1, said powder being free of intentional additives and binders.
 4. The method for making a silicon pellet according to claim 1, said powder comprising an organic binder, and said compacting with pressure comprising adding sufficient heat whereby said binder is evaporated.
 5. The method for making a silicon pellet according to claim 1, wherein said step of dry compressing comprises applying a force greater than or equal to about approximately 10,000 Newton's to said powder. 